Every few
years, the trade winds of the equatorial Pacific slacken, allowing warm water
in the west to slosh eastward and start an El Niño. Related to the ocean
warming are atmospheric changes, called the Southern Oscillation, which disrupt
climate worldwide  bringing drought and fire to some areas and torrential
rain and floods to others. These events, together known as ENSO, may be more
predictable than previously thought, according to a new climate model tested
with a century and a half of data.

These Advanced Very High Resolution Radiometer
images show the 1997-1998 El Niño, with a view of the sea surface before
the event (left), with warm Pacific waters in the west, and during the event
(right), with warm Pacific waters shifted to the east. The bumps are sea-surface
height, and the colors are sea-surface temperature. Red is 30 degrees Celsius
and blue is 8 degrees Celsius. A new model suggests that predicting such events
several years in advance may be possible. Images courtesy NASA.

Dake Chen, an oceanographer at Lamont-Doherty Earth Observatory (LDEO) at Columbia
University, and colleagues tested their new model with historical weather data
collected between 1857 and 2003, the longest period over which any model has
been used to forecast ENSO events. The coupled ocean-atmosphere model successfully
predicted all of the larger ENSO events during that period, but it was not as
capable of predicting the smaller ones, according to their study published in
the April 15 Nature. For some large events, the results showed lead times
of up to two years. Previous successful ENSO models predicted events about six
to nine months in advance.

The model, however, not only predicted those ENSOs that occurred but also some
that did not. The authors results suggest that false alarms are
limiting their models forecast skill at long lead times, says Andrew
Wittenberg, a climatologist with NOAAs Geophysical Fluid Dynamics Laboratory.
He adds that the challenge for all ENSO models, including the LDEO one, will
be to hit the big ENSO events without giving too many false alarms.

Scientists have long struggled to model the complex interactions between the
ocean and atmosphere of the tropical Pacific. Chens co-authors and LDEO
colleagues, Mark Cane and Steve Zebiak, developed the first successful coupled
ocean-atmosphere model, which predicted the 1986-1987 ENSO. Chens recent
study is the latest in the ongoing debate about how predictable ENSO really
is.

On one side, researchers argue that chaotic forces in the atmosphere vary so
much from event to event that long-term prediction may be impossible. On the
other side, researchers say that what really matters are the initial starting
conditions  the sea-surface temperatures and prevailing winds present
at the onset of an ENSO cycle. If ENSO is a self-sustaining oscillation
controlled by internal dynamics, it should be predictable at long lead times,
Chen says.

Because his teams model was able to predict every major ENSO in the last
150 years without accounting for atmospheric chaos, Chen and his colleagues
say that it shows these forces are not as influential as previously thought.
And, if climatologists can correctly determine at what point a particular ENSO
is in its cycle, they may be able to predict future peaks up to two years in
advance.

However, such methods and their results still need further testing, says David
Anderson, a climatologist at the European Centre for Medium-Range Weather Forecasts
in Reading, England. In an accompanying Nature commentary, Anderson questioned
whether the models success in predicting the future (will) match
that of its performance in predicting the past.

Chen says they will soon find out. The team expects to post long-lead forecasts
on the research groups Web site shortly. I believe we should be
able to predict large El Niños two years in advance, he says, though
I am less confident in the predictability of small events.

The outcome of the debate over whether long-term prediction is truly possible
could mean the difference between a warning of a>> few months and one
of a few years for regions of the world affected by the severe weather ENSO
may bring.

Models are essential for understanding and predicting ENSO, which is by
far the largest and most influential short-term climate fluctuation in Earths
climate system, Chen says. Long-lead forecasts of ENSO will not
only benefit the tropical countries that are directly under the influence of
ENSO, but also improve our ability to manage disastrous floods and droughts
in many parts of the world. His model, he says, provides hope for early
forecasting of ENSO events.